The invention relates to the field of producing device quality (Al)InGaP alloys on lattice-mismatched substrates.
Epitaxial graded composition buffers of InxGa1xe2x88x92xP on GaP substrates (InxGa1xe2x88x92xP/GaP) are promising substrates for high performance optoelectronic devices. InxGa1xe2x88x92xP alloys with large bandgaps that are difficult or impossible to achieve lattice-matched to GaAs substrates can be grown on graded buffers, providing direct bandgap emission of the critical green to orange wavelengths that lie between the capabilities of GaN-based and GaAs-based light emitting diode (LED) and laser diode technologies. InxGa1xe2x88x92xP/GaP substrates are also inherently transparent to devices grown on them, which roughly doubles light extraction efficiency in LEDs compared to absorbing substrates such as GaAs. The transparency of InxGa1xe2x88x92xP/GaP has also been used to produce negative electron affinity GaAs and InGaAs photocathodes that operate in transmission mode, and a variety of other optoelectronic detectors and modulators can be envisioned to take advantage of a transparent semiconductor substrate. Furthermore, GaP is nearly lattice-matched to Si, so InxGa1xe2x88x92xP/GaP is one natural choice for integrating compound semiconductor devices on Si substrates.
Graded buffers are grown to efficiently relieve lattice-mismatch strain between substrates and films of differing lattice constants. For most optoelectronic device applications, direct bandgap compositions of InxGa1xe2x88x92xP are desired. The  greater than 2% lattice-mismatch between GaP and direct bandgap compositions of InxGa1xe2x88x92xP results in heavily defective single heterostructures, due to the large and abrupt introduction of strain at one interface. A graded buffer of InxGa1xe2x88x92xP on GaP slowly introduces strain over many interfaces, which minimizes dislocation interactions, maintains a low state of strain, and minimizes dislocation nucleation during growth. Consequently, graded buffers typically have orders of magnitude lower threading dislocation densities than single heterostructures.
The growth of InxGa1xe2x88x92xP/GaP has been studied for decades using a variety of growth techniques, including hydride vapor phase epitaxy (HVPE), gas-source molecular beam epitaxy (GSMBE), and metal-organic vapor phase epitaxy (MOVPE). Early HVPE experiments with InxGa1xe2x88x92xP/GaP and GaAsxP1xe2x88x92x/GaAs established some of the basic principles of dislocation dynamics in graded buffers. Since then, visible LEDs have been demonstrated on InxGa1xe2x88x92xP/GaP. HVPE has been used to produce LEDs operating at wavelengths from 565 nm to 650 nm, however, device efficiency decreases dramatically when InxGa1xe2x88x92xP/GaP is graded beyond xxcx9c0.35. GSMBE has been used to grow InxGa1xe2x88x92P/GaP with photoluminescence (PL) ranging from 560 nm to 600 nm, with decreasing PL intensity in buffers graded beyond xxcx9c0.32.
The agreement of results showing degradation beyond xxcx9c0.3 with two very different growth techniques is striking. Both techniques result in the conclusion that material degradation is a natural consequence of increasing lattice-mismatch, presumably through increasing defect density. This intuitive picture is inconsistent with earlier work, which concluded from experimental and theoretical considerations that strain relaxation in graded buffers is a steady-state process, hence defect density should be constant.
Developments in the GexSi1xe2x88x92x/Si system have provided new insights into dislocation dynamics in graded buffers that can aid in understanding InxGa1xe2x88x92xP/GaP. It has been demonstrated that the formation of dislocation pileups is the primary cause of material degradation with continued grading in GexSi1xe2x88x92x/Si. Since dislocations immobilized in pileups can no longer glide to relieve strain, pileups force the nucleation of additional dislocations to continue the relaxation process. It has been proposed that pileups were caused by an interaction between dislocations and surface roughness. Misfit dislocation strain fields produce surface undulations and gliding dislocations can be pinned in between, which leads to pileups. Surface roughness increases as more dislocations are pinned, resulting in a recursive and escalating cycle of dislocation pinning and surface roughening. It was then showed that controlling surface roughness by periodic planarization can suppress pileup formation in GexSi1xe2x88x92x/Si and recover a steady-state dislocation density between x=0.3 to x=1. The recovery of steady-state dislocation dynamics is compelling evidence that pileup formation due to the interaction of dislocations and surface roughness is responsible for material degradation with continued grading.
Recent work with InxGa1xe2x88x92xP/GaP grown by MOVPE also showed a strong correlation between surface roughness and the density of dislocations and pileups. Pileup formation was tentatively attributed to the proposed mechanisms, but comparison with GexSi1xe2x88x92x/Si results suggests that the sensitivity of defect density to surface roughness is much greater than expected in InxGa1xe2x88x92xP/GaP. Related work with InxGa1xe2x88x92xAs/GaAs noted the presence of xe2x80x9chigh-energy boundariesxe2x80x9d that appeared to pin dislocations, although their overall impact on relaxation was unclear. Defects similar to the xe2x80x9chigh-energy boundariesxe2x80x9d have not been observed in GexSi1xe2x88x92x/Si, so defects of this type may account for the difference in pileup behavior noticed between InxGa1xe2x88x92xP/GaP and GexSi1xe2x88x92x/Si.
In accordance with the invention, the evolution of dislocation dynamics in InxGa1xe2x88x92xP/GaP grown by MOVPE is explored. Starting with the question of escalating defect density a previously unrecognized defect microstructure that causes pileups and dominates dislocation dynamics in InxGa1xe2x88x92xP/GaP is shown by microscopic characterization and macroscopic modeling. The evolution of microstructure in graded buffers is mapped, and its interaction with dislocation dynamics is modeled. By controlling the new defect microstructure, nearly ideal relaxation behavior dominated by dislocation kinetics is also observed. Through analysis and modeling, a proposed kinetic model for relaxation in graded buffers is confirmed. The evolution of branch defects is used to explain the microstructure of both indium-bearing phosphides and arsenides over a wide range of conditions. The new understanding and control of dislocation dynamics and microstructure are used to derive a set of design rules and an optimization strategy for high quality graded buffer growth. A simple process optimization results in material with a dislocation density of 4.7xc3x97106 cmxe2x88x922 at x=0.39.
Accordingly, the invention provides a method of forming a semiconductor structure including providing a single crystal semiconductor substrate of GaP, and fabricating a graded composition buffer including a plurality of epitaxial semiconductor Inx(AlyGa1xe2x88x92y)1xe2x88x92xP alloy layers. The buffer includes a first alloy layer immediately contacting the substrate having a lattice constant that is nearly identical to that of the substrate, subsequent alloy layers having lattice constants that differ from adjacent layers by less than 1%, and a final alloy layer having a lattice constant that is substantially different from the substrate. The growth temperature of the final alloy layer is at least 20xc2x0 C. less than the growth temperature of the first alloy layer.